Ship an element reference skill for Boost

[simonhamp]

Adds an element reference to the Boost files this plugin ships, so agents don't have to read vendor source to find props. It's the mobile-ui half of NativePHP/mobile-air#517.

In the stack benchmark, a bare NativePHP run took 67 to 77 turns and $1.70 to $2.10 per trial because the model doesn't know SuperNative or these elements. With a short guide as CLAUDE.md it took 14 turns, $0.38 and 3 minutes. Most of the guide's element knowledge belongs here.

What's added

A new nativephp-mobile-ui skill (resources/boost/skills/nativephp-mobile-ui/SKILL.md, 117 lines):

• The events and how handler arguments work.
• A table of the common elements and the props that matter: layout, text, button, the three text inputs, list, icon, top-bar.
• Text inputs: .debounce.300ms, taking the text from @submit, keep-focus-on-submit being iOS only.
• List items: the camelCase props, leading and trailing slots, on-trailing-press (from Read on-trailing-press on list items #108), swipe actions, and a worked example.
• A custom row for when a list item can't do it (strikethrough).
• Layout notes and what the elements look like in Native::test() trees.

The always-loaded guideline gets 7 lines: a pointer to the skill, the three text input tag names with the binding advice, and the list-item camelCase and swipe rules.

How I checked it

• In a copy of mobile-starter main with this branch and the mobile-air branch as path repos, I rendered the skill's three Blade examples unchanged in one screen and drove them with Native::test(): checkbox change, trailing button, swipe action, row press and @submit all reach the right method with the right arguments.
• The props listed are the ones applyAttributes() reads on main. <native:text-input> throws "Unknown native element type: text_input".
• php artisan boost:install --no-interaction renders the guideline into AGENTS.md with no Blade errors and installs the skill unchanged.
• composer test: 264 passed, 3 failed. The same 3 ListItemImageSourceTest tests fail on main (the lock file pins nativephp/mobile 4.0.0-rc.1).

Lines to update when these merge

• Add headline-line-through to list items #109 (headline-line-through): add it to the list item props and change "A list item can't strike through or restyle its headline" to use it. The custom row example can stay for rows with their own buttons.
• Stop stale model echoes from overwriting newer text in text inputs #110 (stale echo): the debounce advice becomes a tip, not a workaround. Soften "drops or reorders characters" in the skill and in the guideline.
• Implement keep-focus-on-submit on Android and report focus state for press dispatch #53 (Android keep-focus-on-submit): drop "only works on iOS".

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